The invention relates to integrated optical devices, and in particular, to polarization couplers.
A known method for amplifying an optical signal beam propagating in a waveguide is to pass a pump beam through the same waveguide. In this method, known in the art as “Raman amplification,” the pump beam transfers energy to phonons within the waveguide. If the wavelength of the pump beam is correctly chosen, the energy in these phonons is transferred to the signal beam, thus amplifying the optical signal.
In most cases, the optical signal occupies a band of wavelengths. A single pump beam can only amplify a limited portion of the entire band occupied by the optical signal. As a result, it is often necessary to combine several pump beams, each at a slightly different wavelength, to provide amplification over the entire band occupied by the optical signal.
The extent to which a pump beam amplifies the signal beam also depends on the polarization difference between the pump beam and the signal beam. The pump beam, having been generated by a nearby laser, is typically linearly polarized. The signal beam, having been generated far away, has become thoroughly depolarized. As a result, the amplification of the signal beam will depend on whether the linearly-polarized pump beam and the randomly polarized signal beam happen to share the same polarization.